We propose to determine X-ray crystallographically the structures of a number of proteins involved in blood coagulation, particularly that of prothrombin fragment 1, in order to: 1) obtain the three dimensional folding of the kringle sequence, 2) determine the manner in which Ca+2 ions bind to the gamma-carboxyglutamic acid residues (Gla) of prothrombin, 3) fix the conformational changes accompanying Ca+2 ion binding, 4) establish the role of the carbohydrate with aglyco-fragment 1 and that of the Gla residues in the interaction with negatively charged phospholipid and 5) determine the differences in structure between human and bovine sources and relate them to known differences in primary structure. Since Ca+2 ion binding is pH dependent, we also propose to investigate the structure of fragment 1 versus these two variables.